1. Field of the Invention
This invention relates to an electrostatic coating process and an apparatus for use therein. More specifically, it relates to an improvement in a process and an apparatus for performing electrostatic coating by atomizing a liquid paint in the electrically charged state from a high-velocity rotating member having a high voltage applied thereto.
2. Description of the Prior Art
For electrostatically atomizing a liquid paint, a method has previously been known which comprises feeding a liquid paint in thin film form toward the circumferential edge of a rotating member along the inner circumferential surface thereof and discharging the paint in atomized form from a knife-like tip provided on the circumferential edge of the rotating member. This electrostatic atomizing method, however, is not entirely satisfactory. When the viscosity of the liquid paint is relatively low and the rotating speed of the rotating member is as low as several hundred to several thousand revolutions per minute or the amount of the paint discharged is relatively small, the resulting mist of the paint will assume a satisfactory state. But when the viscosity of the paint is relatively high, or the amount of the paint discharged is increased by increasing the rotating speed of the rotating member, the paint tends to leave the knife-like edge in an irregular liquid film form, and this tendency leads to the defect that the mist of the paint includes air and the resultant coated film formed on an article contains bubbles.
On the other hand, the use of a liquid paint having the highest possible viscosity by decreasing the amount of the solvent is very desirable from the standpoint of the saving of the cost of the solvent and the cost of coating and baking, the prevention of air pollution, the expediting of the coating treatment, etc.
An attempt has already been made to solve the problem of air bubble entrainment which arises when a liquid paint having a high viscosity is used and the rotating speed of an atomizing rotating member is increased. For example, Japanese Patent Publication No. 41825/1980 discloses a method for feeding a liquid paint as a thin film-like stream toward a circular releasing edge having a knife edge-like sectional shape in a rotating atomizing device, in which the paint is divided into a number of branched streams and fed to the circular releasing edge by providing in the peripheral edge portion of a paint-guiding surface of the rotating atomizing device a number of shallow depressed grooves extending in the advancing direction of the paint stream and reaching the outside end of the aforesaid peripheral edge portion. According to this known method, the formation of a liquid film extending outwardly beyond the releasing end edge will be inhibited when the amount of the liquid paint discharged per unit time is relatively low. However, when the amount of the liquid paint discharged is relatively large, the size of the resulting paint droplets will become large, or air bubble entrainment will be liable to occur.